Mobile radio transceivers typically derive their operating frequencies with a phase locked loop frequency synthesizer. In such a synthesizer, a phase locked loop circuit controls a variable frequency oscillator, i.e., a voltage controlled oscillator (VCO), to lock in at the frequency and phase angle of a standard or reference frequency. In this fashion, the VCO will have the same accuracy as the standard. The phase locked loop operates to track the operation of the reference oscillator.
In general, a phase locked loop includes a VCO, a reference oscillator, a phase detector and a loop filter. The phase detector compares the phase of the VCO output signal with the phase of the signal from the reference oscillator. The phase detector generates an error signal related to the difference between the two compared signals. This output signal is processed by a low pass filter and is then applied as a control signal to an input terminal of the VCO to control the frequency of the oscillator output signal.
As in many other electronic technologies, over the last decade the competitive marketplace and the consuming public continuously demand mobile radios which include more functions but which are smaller and less expensive than currently existing radios. With respect to mobile radio size, as cars have tended to become smaller, the need has arisen for mobile radios which are also more compact. Accordingly, developments in the mobile radio communications art which eliminate redundant circuitry or which otherwise permit a radio to be made smaller or to be manufactured at a lower cost are extremely significant developments.
The present invention serves to eliminate redundant mobile radio circuitry by uniquely implementing three functions which heretofore have been implemented as discrete functions requiring two or more separate frequency synthesizer ports. More specifically, the present invention uses one VCO port to input signals for introducing modulation, centering the VCO operating frequency range and for introducing a negative bias voltage for optimally biasing the VCO tuning varactor. These functions are implemented so as to reduce the cost and complexity conventionally associated with providing extra circuits and pins for implementing related functions.
In the prior art, modulation signals are generally applied to the VCO at a separate port and are typically introduced across a dedicated varactor to achieve frequency modulation of the VCO frequency. In synthesizers designed for land mobile radios, the closed loop bandwidth of the synthesizer creates a high pass modulation response with decreasing deviation below the loop bandwidth for signals introduced into the VCO. Several trade-offs are balanced in the selection of the exact bandwidth, but in most cases involving frequent channel changes, the bandwidth is between 20 and 100 Hz.
The common use of frequency dividers in phase locked loops to create locking of the VCO to a single reference frequency creates a change in the loop bandwidth as different channels are selected. As a result, it is difficult to obtain consistent deviation for frequencies at or below the frequency bandwidth.
One mechanism that has been utilized to obtain a flat modulation response is to modulate both the reference oscillator and the VCO. It is, however, undesirable in many applications to modulate the reference source due to increased phase locked loop complexity and associated extra costs.
As will be explained in detail below, the present invention instead of introducing modulation at several points in the phase locked loop and instead of utilizing a dedicated modulation input port, introduces modulation into the system to create a substantially flat frequency response at the same VCO port at which VCO frequency centering and negative bias control signals are introduced.
Frequency centering is an important adjustment which permits an operator to center the VCO in the range over which the radio is designed to operate. In this regard, when a mobile radio is manufactured, the radio is typically not set at an optimum frequency at the precise center of the operating range.
Most VCO's need to be set to a predetermined center frequency to insure proper operation. The methods typically utilized in conventional radios to set the radio to a particular frequency involve using (1) a tunable inductor having a solenoid winding with a metal, ferrite, or powdered iron core (or a helical coil which is tuned by an adjustable element whose proximity to the coil changes its resonant frequency), (2) a tunable capacitor, or (3) a microstrip line whose physical characteristics may be mechanically adjusted.
These prior art techniques for centering the VCO frequency suffer from significant disadvantages. In this regard, the movable elements of a tunable inductor or tunable capacitor are potential sources of unwanted modulation in the VCO since mechanical vibration applied to the oscillator can move the adjustable components creating a frequency shift related to the frequency vibration. Microstrip lines are susceptible to detuning by changes in proximity of surrounding components and shields.
In the present invention, a potentiometer is utilized both for providing a negative bias to the varactor in the VCO resonant circuit and for setting the VCO to the appropriate center frequency. Moreover, the signals which provide the appropriate VCO frequency centering and negative bias signals to the VCO are introduced at the same VCO port at which the modulation input signal is introduced to the VCO.
In conventional radios, the control voltage which tunes the oscillator and which is typically coupled to the cathode of a VCO tuning varactor is typically utilized to reverse bias the VCO tuning varactor(s). The anode of the varactor in such a configuration is grounded. The present invention provides an adjustable negative bias voltage to the anode of the VCO tuning varactor diode to force the diode to consistently operate at a minimum of 1 volt reverse bias which is a more desirable negative bias voltage than is typically provided by the VCO control voltage.